When metalliferous ore deposits are formed in the Earth's crust they are rarely composed of just one metalliferous element; indeed it is well documented that certain combinations of metals commonly occur together. For example lead deposits are commonly associated with zinc and so as the lead concentrate is formed at the mine processing plant, zinc is inherently present in the concentrate as an impurity. It is not until the primary metal is won from the concentrate at a smelter that the impurities are removed.
Many methods are known for extracting metals and their compounds from ores, including methods which involve liberating the metal in the form of its chloride. Such methods have not previously been applied to the recovery of metals such as zinc from slag wastes, reflecting the very different nature of these materials.
Due to the high energy nature of their formation, slag wastes are generally very stable and so do not readily release metal constituents present. In general, therefore, metal impurities entrapped within the slag waste from the smelter are rarely regarded as a commodity and only low capital processing techniques, such as hydrometallurgical processes, have been exploited in order to realise their value. However, these processes are not selective and this indiscriminate dissolution of all the waste often creates further complications in realising the value of the material. A particular problem is the ready leaching of iron that commonly occurs in much greater volume than other metals of interest.
There therefore remains a continuing need for the development of further, improved, commercially viable methods for recovering metals of interest from slag waste material.